This invention relates to improved optical torque sensors especially, but not exclusively, for incorporation in power-assisted steering systems for vehicles.
In our European patent application No. 93300792.4, a torque sensor is described which comprises an input member, an output member, emitter means, first receiver means adapted to receive a first signal, second receiver means adapted to receive a second signal, and signal processing means, in which said signal processing means is adapted to receive output signals from the first and second receiver means, the output signals of the receiver means being dependent upon the first and second signals which they receive, and the signal processing means being adapted to process the output signals from the first and second receiver means so as to produce a modified output signal indicative of the relative angular displacement between, or torque applied between, the input and output member.
In a described embodiment of the invention input and output masks are associated with the input and output members respectively, interposed between the emitter means and the first and second receiver means. In this embodiment each of the input and output masks comprises an outer arc of angularly spaced apertures and a radially spaced inner arc of angularly spaced apertures, and preferably the inner and outer arcs of the apertures extend around a complete circle, forming an inner track of apertures and an outer track of apertures. The input and output masks move angularly with the input and output members respectively and are both centred on their common axis of rotational symmetry. The first and second receiver means receive signals through the superimposed inner and outer tracks of apertures respectively, and may each comprise first and second effective receivers respectively.
However, in a torque sensor of this construction, with one or more effective receivers for each track, a problem may be encountered if the two rotating masks are not accurately concentric. Errors in the concentricity of the masks can give rise to a variation in the torque output of the sensor as the input member is rotated at a given torque, the period of the variation being equal to one revolution of the input member. This variation, or "low-frequency ripple" can cause difficulty in determining the zero position of the sensor, since even quite small alignment errors can cause variations of 10% in the sensor output.
The ripple occurs due to periodic variations in the signals from both the inner and outer track receiver means, these variations being out of phase by an amount dependent on the phase relationship between the apertures of the two tracks. Where the two signals are out of phase, the fluctuation is not cancelled out when the torque is calculated, thus a ripple results in the calculated torque signal.
An aim of the present invention is to provide an improved torque sensor which incorporates means to prevent any ripple from being present in the torque signal.